The present invention relates to an ultrasound diagnosis system adapted for medical diagnosis and particularly relates to an ultrasound diagnosis system in which sound velocity set up in respective elements of an ultrasonic probe can be changed in order to eliminate the influence of an inhomogeneous medium.
In the ultrasound diagnosis system, a delay time distribution is given to a plurality of signals received by a plurality of arrayed elements of a probe to form an ultrasonic beam having directivity in a predetermined direction to thereby obtain a sectional image of a testing object. However, because the testing object (human body) is generally an inhomogeneous medium, it is necessary that the delay time distribution is changed in accordance with the testing object in order to form a high-resolution ultrasonic beam. FIG. 2 shows an example of the wavefront of reflected ultrasound pulse in the case where the testing object is homogeneous and in the case where the testing object is inhomogeneous. Now, consideration is made only on the case of reception for the sake of simplification of explanation. When the sound velocity in a medium is known and the medium is homogeneous, the wavefront of ultrasound pulse reflected from a reflecting object 20 reaches elements 21 to 25 (E1 to E5) of a probe as an ideal wavefront 26. In this occasion, the pulse reaches the element 23 (E3) first and the pulse reaches the elements 21 (E1) and 25 (E5) last on the basis of the positional relations between the reflecting object 20 and the elements 21 to 25. Therefore, suitable delay times are given to the pulses received by the elements 22, 23 and 24 (E2, E3 and E4) respectively to adjust the phases of all the pulses. After the phases of all the pulses are adjusted in the aforementioned manner, these pulses are added up to amplify only receiving pulses obtained from a target direction to thereby form a high-resolution sectional image. In the case where the sound velocity in the testing object is known and the testing object is a homogeneous medium, the delay times to be given can be obtained analytically. When the respective distances between the reflecting object 20 and the elements 21 to 25 (E1 to E5), the values of sound velocity to be set up in the respective elements 21 to 25 (E1 to E5) (hereinafter simply referred to as "set-up of sound velocity") under the consideration of difference in propagating path of ultrasound wave and the delay times to be given to the respective receiving signals of the elements are represented by L.sub.i (1.ltoreq.i.ltoreq.5), c.sub.1 (1.ltoreq.i.ltoreq.5) and .tau..sub.i (1.ltoreq.i.ltoreq.5) respectively and when the maximum in L.sub.i /c.sub.i (1.ltoreq.i.ltoreq.5) is further represented by (L/c)max, the following expression (1) is obtained. EQU .tau..sub.i =(L/c)max-(L.sub.i /c.sub.i) (1)
When the distance between the reflecting object and the probe and the pitch of the elements are represented by R and d respectively, the following expressions (2) to (4) are obtained in the case of FIG. 2. ##EQU1##
In the case where the sound velocity in the testing object is known and the testing object is a homogeneous medium, the set-up of sound velocity c.sub.i is common to the elements 21 to 25 (E1 to E5) regardless of i. In practice, however, an inhomogeneous medium 28 is present between the reflecting object 20 and the elements 21 to 25, so that the wavefront of an ultrasound pulse is formed as an aberrated wavefront 27. Accordingly, it is necessary to correct the delay times to be given to the receiving signals of the respective elements. First, the ideal wavefront 26 can be made to approach the aberrated wavefront 27 by changing the set-up of sound velocity c.sub.i for all the elements regardless of i. However, this is rough correction. In order to obtain a high-resolution sectional image, it is necessary to correct under the consideration of the change of sound velocity from the point of view of averaging of velocity differences between the elements on the propagating path of ultrasound wave. As a conventional example posterior to the method of changing of a sound velocity in common with each element, JP-A-59-212791 discloses a method of changing sound velocities to be set up in the respective elements.